1. Field of the Invention
This invention relates generally to tempering glass sheets and, more particularly, to tempering glass sheets using a multi-stage tempering process.
2. Description of the Current Technology
It is known to temper glass sheets to increase the strength or breaking resistance of the glass. Traditionally, this tempering is done either by chemical tempering or thermal tempering. In chemical tempering, relatively small ions, such as sodium, are replaced by larger ions, such as potassium, or smaller ions, such as lithium, are replaced by larger ions, such as sodium and/or potassium. The crowding of the larger ions into the spaces left by removal of the smaller ions produces a compression of the surface layers of the glass.
In thermally tempered glass, glass sheets are heated to an elevated temperature above the glass strain point near the glass softening point and then are chilled to cool the glass surface regions relatively rapidly while the inner regions of the glass cool at a slower rate. This differential cooling results in a compressive stress in the glass surface regions balanced by a tension stress in the interior of the glass. The resultant tempered glass has a much greater resistance to fracture than untempered glass. Also, in the event that the tempered glass does fracture, its breakage pattern is significantly different than that of untempered glass. Tempered glass typically shatters into small fragments which become smaller as the temper increases. Because the glass breaks into small fragments, it is less likely to cause injury due to laceration. Untempered glass typically fractures to form large pieces having sharp edges.
In a conventional thermal tempering process, the heated glass sheet is conveyed through a cooling chamber or “quench” in which the glass sheet is cooled rapidly from an initial furnace exit temperature, typically in the range of 1160° F. to 1300° F. (627° C. to 704° C.), to a quench exit temperature, typically in the range of 900° F. to 950° F. (482° C. to 510° C.), at which temperature the stresses (compression and tension) in the glass become permanently set. The actual temperature ranges utilized in the process are glass composition dependent. The glass viscosity, which is temperature dependent, along with other glass physical properties are the determining factors for setting process requirements.
In U.S. Pat. No. 4,913,720 to Gardon et al., glass sheets are tempered at a first cooling station with a first rate of heat transfer and then moved to a second cooling station to be cooled at a second rate of heat transfer, with the second rate of heat transfer being less than the first rate. The time is adjusted such that this process initially cools the surface of the glass sheet below the strain point, leaving the center above the strain point, after which the cooling rate is reduced. Thereafter, the second cooling rate cools both the center and the surface below the strain point. This modified tempering process results in tempered glass that mimics ion exchange glass. That is, the center tension is low and therefore the glass fractures into large pieces rather than small pieces as in conventionally tempered glass. Such glass is particularly useful for aircraft windshields so that even in the event of glass fracture, the pilot can more easily see through large pieces of broken glass rather than very small pieces of broken glass produced by the conventional tempering process. The Gardon process results in glass sheets having a high surface compression but a low center tension.
As a general rule, the higher the temper level, e.g., the higher the surface compression and center tension, the stronger or more fracture resistant is the glass sheet. Therefore, it would be advantageous to provide an apparatus and method for increasing the temper level of glass sheets above the temper level available with conventional tempering techniques. That is, to produce glass having relatively high surface compression as well as high center tension. Such uses include weather resistant (hurricane and typhoon resistant) windows, countertops or furniture surfaces, glass partitions (such as sporting partitions, e.g., hockey arena glass), stronger and/or lighter architectural, automobile or aircraft glass. For example, current hurricane resistant glass is typically formed from two sheets of heat strengthened or annealed glass that is laminated together with polyvinylbutyral. The polyvinylbutyral layer typically costs more than the two glass sheets. If this laminated structure could be replaced by a single, highly tempered glass sheet, weight and cost could be reduced. However, current thermal tempering processes do not lend themselves to such high tempering levels due to the processing methods used. Therefore, it would be advantageous to provide an apparatus and/or method that could be utilized to provide more highly tempered glass sheets than can be produced by conventional tempering processes.